Touchscreens are input devices for interactive computer systems. They are increasingly used commercially for applications such as information kiosks, order entry systems for restaurants, etc.
The dominant touchscreen technologies are resistive touchscreens, capacitive touchscreens, and acoustic touchscreens. Acoustic touchscreens, i.e., ultrasonic touchscreens, are particularly advantageous when the application demands very durable touch sensitive surface and minimal optical degradation of the displayed image.
Various types of ultrasonic transducers are known. The most common types used in acoustic touchscreens are wedge transducers and direct coupling between a piezoelectric transducer element and the touch substrate. A transducer is a physical element or set of elements which converts energy from one form to another. This includes converting between acoustic wave modes and converting between electrical and acoustic energy. Typically used piezoelectric transducers are formed of a rectangular prismatic piezoelectric ceramic having conductors formed on the surface, which are acoustically coupled to a surface by mounting a flat surface of the ceramic element or metallic electrode formed on the surface flush with a surface of a substrate element, for example the wedge material.
A wedge transducer induces surface-bound waves or plate waves into a substrate. The wedge transducer utilizes the phenomenon that acoustic waves are refracted when they are obliquely incident on a boundary surface of different media. A typical wedge transducer typically consists of a plastic wedge, having a piezoelectric element mounted on a one side, and the hypotenuse adhered to the substrate, which is for example glass. The piezoelectric element couples to a bulk wave in the wedge material. This bulk wave propagates at the critical angle, i.e., the "wedge angle", to refract to or from a horizontally propagating wave in the glass. The wedge material is chosen to have a bulk wave acoustic velocity that is slower than the phase velocity of the desired mode in the touch substrate; the cosine of the wedge angle equals the ratio of these two velocities. Wedge transducers may thus be used for both transmitting and receiving Rayleigh waves, Love waves, and plate waves such as Lamb waves.
In contrast, direct-coupling or "edge" transducers typically provide a piezoelectric element that is directly bonded to the touchscreen substrate in such a fashion that an acoustic wave with appreciable power at a surface of the substrate is directly generated. The interface thus serves the mechanical function of connecting the piezoelectric element to the substrate, as well as the acoustic function of coupling to the desired acoustic mode. FIG. 2B of U.S. Pat. No. 5,162,618, incorporated herein by reference, illustrates an edge transducer used to launch Lamb waves into a thin substrate. See also, U.S. Pat. No. 3,893,047, Lardat. Edge transducers are most naturally used to couple to plate waves with no nodes as a function of depth in the substrate. Some work has been done to develop edge transducers that couple to Rayleigh waves. See Ushida, JP 08-305481 and JP 08-305482, incorporated herein by reference. While such an edge transducer is compact, this leaves the piezoelectric transducer unprotected.
One type of known acoustic touch position sensor includes a touch panel or plate having an array of transmitters positioned along a first edge of a substrate for simultaneously generating parallel surface bound or plate waves that directionally propagate through the panel to a corresponding array of detectors positioned opposite the first array on a second edge of the substrate. Another pair of transducer arrays is provided at right angles to the first set. Touching the panel at a point causes an attenuation of the waves passing through the point of touch, thus allowing interpretation of an output from the two sets of transducer arrays to indicate the coordinates of the touch. This type of acoustic touch position sensor is shown in U.S. Pat. No. 3,673,327 and WO 94/02911, Toda, incorporated herein by reference. Because the acoustic wave diverges, a portion of a wave emitted from one transmitting transducer will be incident on a set of receiving transducers, allowing finer discrimination of touch position than a simple one-to-one relation of transmit and receive transducers would allow. These systems require a large number of transducers.
A commercially successful acoustic touchscreen system, termed the Adler-type acoustic touchscreen, as show in FIG. 1, efficiently employs transducers, by spatially spreading the signal and analyzing temporal aspects of perturbation as indicative of position. A typical rectangular touchscreen thus includes two sets of transducers, each set having a different axis aligned respectively with the axes of a physical Cartesian coordinate system defined by a substrate. An acoustic pulse or pulse train is generated by one transducer, propagating as, e.g., a narrow Rayleigh wave along an axis which intersects an array of reflective elements, each element angled at 45.degree. and spaced corresponding to an integral number of wavelengths of the acoustic wave pulse. Each reflective element reflects a portion of the wave along a path perpendicular to the axis, across a broad region of the substrate adapted for touch sensing, to an opposing array and transducer which is a mirror image of the first array and transducer, while allowing a portion to pass to the next reflective element of the array. The transducer of the mirror image array receives an acoustic wave consisting of superposed portions of the incrementally varying wave portions reflected by the reflective elements of both arrays, directed antiparallel to the emitted pulse. The acoustic waves are thus collected, while maintaining the time dispersion information which characterizes the axial position from which an attenuated wave originated. Wavepaths in the active region of the sensor have characteristic time delays, and therefore a wavepath or wavepaths attenuated by an object touching the touch sensitive region may be identified by determining a timing of an attenuation in the composite returning waveform.
A second set of arrays and transducers are provided at right angles to the first, and operate similarly. Since the axis of a transducer corresponds to a physical coordinate axis of the substrate, the timing of an attenuation in the returning wave is indicative of a Cartesian coordinate of a position on the substrate. The coordinates are determined sequentially to determine the two dimensional Cartesian coordinate position of the attenuating object. The system operates on the principle that a touch on the surface attenuates surface bound or plate waves having a power density at the surface. An attenuation of a wave traveling across the substrate causes a corresponding attenuation of waves impinging on the receive transducer at a characteristic time period. Thus, the controller need only detect the temporal characteristics of an attenuation to determine the axial coordinate position. Measurements are taken along two axes sequentially in order to determine a Cartesian coordinate position. See, U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914 and 4,791,416, Re. 33,151 incorporated herein by reference. U.S. Pat. No. 4,642,423, to Adler, incorporated herein by reference, addresses pseudo-planarization techniques for rectangular touchscreen surfaces formed by small solid angle sections of a sphere.
As shown in FIG. 1, the system transmits a short-time ultrasonic wave signal in the form of a burst by acoustic wave transmitting means 11 and 12, disperses the transmitted signal to the whole surface of a coordinate input range 15 through reflecting members 13 and 14, which act as acoustic wave dispersers. The system receives the signal by receiving means 18 and 19 through reflecting members 16 and 17, which act as acoustic wave condensers, and analyzes the received signal along the time base, to detect indicated coordinates.
A portion of the touchscreen system where the wedge type transducer is located on the surface of the panel is inevitably higher than the surface of the panel. As shown in FIG. 2, when a display is composed of a curved panel such as a typical cathode-ray tube, a space where a wedge-type transducer 23 can be located often appears between a curved panel 21 and a bezel 22 covering the periphery of the curved panel 21. When the display is composed of a flat panel, such as a liquid crystal display or a plasma display as shown in FIG. 3, however, there is no clearance between a panel 24 and a bezel 25 in the periphery of the surface of the panel 24 covered with the bezel 25, whereby there is no room for location of the wedge-type transducer. When a wedge-type transducer is employed, therefore, the ultrasonic type touch panel is not well adapted for use with a flat panel. Thus, the type of applicable displays and housing configurations that may be adapted are greatly restricted.
The known reflective arrays are generally formed of a glass frit that is silk-screened onto a soda-lime glass sheet, formed by a float process, and cured in an oven to form a chevron pattern of raised glass interruptions. These interruptions typically have heights or depths of order 1% of the acoustic wavelength, and therefore only partially reflect the acoustic energy. In order to provide equalized acoustic power at the receiving transducer, the spacing of the reflective elements may be decreased with increasing distance from the transmitting transducer, or the balance of acoustic transmissivity and reflectivity of the reflective elements may be altered, allowing increased reflectivity with increasing distance from the transmitting transducer. Because the touch sensor is generally placed in front of a display device, and because the reflective array is generally optically visible, the reflective arrays are generally placed at the periphery of the substrate, outside of the active sensing area, and are hidden and protected under a bezel.
In order to further reduce the number of transducers, folded acoustic paths may be employed. FIG. 11 of U.S. Pat. No. 4,700,176 teaches the use of a single transducer for both transmitting the wave and receiving the sensing wave, with a single reflective array employed to disperse and recombine the wave. Such systems therefore employ a reflective structure opposite the reflective array. Thus, the acoustic wave may be reflected 180.degree. off an edge of the substrate or an array of reflectors parallel to the axis of the transmission reflective grating and reflected back through the substrate to the reflective array and retraces its path back to the transducer. The transducer, in this case, is time division multiplexed to act as transmitter and receiver, respectively, at appropriate time periods. A second transducer, reflective array and reflective edge are provided for an axis at right angles to allow determination of a coordinate of touch along perpendicular axes. A "triple transit" system, provides for a single transducer which produces a sensing wave for detecting touch on two orthogonal axes, which both produces and receives the wave from both axes. See, U.S. Pat. Nos. 5,072,427, 5,162,618, and 5,177,327, incorporated herein by reference. The vast majority of present commercial products are based on Rayleigh waves. Rayleigh waves maintain a useful power density at the touch surface due to the fact that they are bound to the touch surface. A Rayleigh wave is a wave having vertical and transverse wave components with substrate particles moving along an elliptical path in a vertical plane including the axis of wave propagation, and wave energy decreasing with increasing depth in the substrate. Both shear and pressure/tension stresses are associated with Rayleigh waves.
Mathematically, Rayleigh waves exist only in semi-infinite media. In realizable substrates of finite thickness, the resulting wave may be more precisely termed a quasi-Rayleigh wave. Here, it is understood that Rayleigh waves exist only in theory and therefore a reference thereto indicates a quasi-Rayleigh wave. For engineering purposes, it is sufficient for the substrate to be 3 or 4 Rayleigh wavelengths in thickness in order to support Rayleigh wave propagation over distances of interest to touchscreen design.
In addition to Rayleigh waves, acoustic waves that are sensitive to touches on the surface, i.e., a touch on the surface leads to a measurable attenuation of acoustic energy, include but are not limited to Lamb, Love, zeroth order horizontally polarized shear (ZOHPS), and higher order horizontally polarized shear (HOHPS). See, U.S. Pat. Nos. 5,591,945, 5,329,070, 5,260,521, 5,243,148, 5,177,327, 5,162,618 and 5,072,427, incorporated herein by reference.
Like Rayleigh waves, Love waves are "surface-bound waves", i.e. waves bound or guided by one surface and unaffected by the substrates other surface provided the substrate is sufficiently thick. In contrast to Rayleigh waves, particle motion for Love waves is horizontal, i.e. parallel to touch surface and perpendicular to the direction of propagation. Only shear stress is associated with a Love wave.
Another class of acoustic waves of possible interest in connection with acoustic touchscreens are plate waves. This includes the horizontally polarized shear plate waves of lowest (ZOHPS) and higher orders (HOHPS), as well as Lamb waves of various symmetries and orders.
It is known that arrays of reflective elements having a regular spacing or spacing increment can diffract or scatter incident radiation, including acoustic waves. The known Adler-type touchscreen design, discussed above, employs a reflective array to coherently reflect an acoustic wave at a predetermined angle. The touchscreen designs according to U.S. Pat. Nos. 5,072,427 and 5,591,945, expressly incorporated herein by reference, extend this principle, providing a reflective array which coherently reflects an acoustic wave at a predetermined angle on the surface while converting a wave mode of the wave. Thus, it is known that the interaction of an acoustic wave with a diffraction grating can convert wave energy between various wave modes.
The touches sensed by the acoustic waves may include a finger or stylus pressing against the surface directly or indirectly through a cover sheet. See, e.g., U.S. Pat. No. 5,451,723, incorporated herein by reference, which employs a shear mode wave acoustic sensor system and edge transducers. The use of wedge transducers, often used in Rayleigh wave acoustic touchsensors, makes mounting of a cover sheet on the front surface difficult, due to mechanical interference between the coversheet and the wedge transducers. As with LCD touchmonitor design, use of wedge transducers complicates mechanical design and may limit options.
One approach to address such mechanical interferences from wedge transducers is described in U.S. patent application Ser. No. 08/610,260, filed Mar. 4, 1996, expressly incorporated herein by reference. As disclosed herein, a wedge transducer may be mounted on a front surface bevel adjacent to the touch region, which recesses the wedge transducer behind the front surface of the touchscreen substrate, but incurs acoustic losses. Contrary to the needs of liquid crystal display (LCD) touchmonitor design, such designs typically add border width to the touchscreen.
Masao Takeuchi and Hiroshi Shimizu, "Theoretical analysis of grating couplers for surface acoustic waves", Journal of the Acoustic Society of Japan, 36(11):543-557 (Jun. 24, 1980), incorporated herein by reference, discloses a grating transducer and the theoretical framework of their operation. See also, Published research paper of Masao Takeuchi and Hiroshi Shimizu of Tohoku University on "Unidirectional excitation of plate waves in a periodic structure" (in Japanese) (1991). See, also J. Melngailis and R. C. Williamson, "Interaction of Surface Waves and Bulk Waves in Gratings: Phase shifts and Sharp Surface Wave/Reflected Bulk Wave Resonances", Proc. 1978 IEEE Ultrasonics Symposium, p. 623; Herman A. Haus, Annalisa Lattes and John Melngailis, "Grating Coupling between Surface Acoustic Waves and Plate Modes", IEEE Transactions on Sonics and Ultrasonics, p. 258 (September, 1980).
In a wedge transducer, unconverted bulk wave from the piezoelectric transducer that is not coupled to, e.g., Rayleigh waves does not enter the touchscreen substrate and is dissipated in the wedge material. In contrast, in a surface grating arrangement, bulk wave energy from the piezo that is not converted to, e.g., Rayleigh waves at the grating will take the form of parasitic bulk or plate waves propagating in the substrate material itself.
As made clear from Takeuchi et al. (1980), a theoretical upper limit of conversion efficiency for incident bulk-wave energy to Rayleigh-wave energy is 81%, leaving a theoretical minimum of 19% of the bulk-wave energy in the form of parasitic waves Even this efficiency is difficult to achieve in practice; see discussion of "F factor" in Takeuchi et al. (1980). It is thus clear that a grating transducer has a significant disadvantage relative to wedge transducers: strong generation of parasitic waves. For typical applications of ultrasonic transducers, such as non-destructive testing, such strong generation of parasitic waves is often unacceptable. Even in touchscreens, the prospect of significant parasitic waves generated propagating parallel in the plane of the substrate to the desired wave would be considered troublesome. Similar considerations apply to the sensitivity of receive-mode grating transducers to parasitic waves.
It is known that undesired parasitic waves can be a problem for at least some examples of acoustic touchscreen design. For example, see FIGS. 13, 14 and 17, and associated text, of U.S. Pat. No. 5,260,521, the entirety of which is expressly incorporated herein by reference. Touch recognition algorithms in commercial touchscreen controllers require that the desired signal be free from interference from parasitic signals.
R. F. Humphryes and E. A. Ash, "Acoustic Bulk-surface-wave transducer," Electronics Letters (Volume 5 No. 9) May 1, 1969 includes discussion of a grating transducer that employs asymmetrical grating teeth as a means to construct a unidirectional transducer. This reference also considers a pair of gratings on opposing substrate surfaces as a means to transfer Rayleigh waves between surfaces.
U.S. Pat. No. 5,400,788, FIGS. 12, 13, and 14, the entirety of which is expressly incorporated herein by reference, discloses a transducer arrangement in which gratings are used to couple Rayleigh waves to bulk waves. Interdigital transducers on a piezoelectric substrate generate Rayleigh waves that are then converted via gratings to pressure bulk waves that are then coupled into an acoustic wave-guide (which optionally is also an optical fiber). The interdigital electrodes and the gratings form sections of circular arcs.
U.S. Pat. No. 5,673,041, "Reflective mode ultrasonic touch sensitive switch," the entirety of which is expressly incorporated herein by reference, discloses an ultrasonic touch sensor that makes use of a thickness mode resonance of a touch panel substrate. An array of transparent piezoelectric elements, formed for example of polyvinylidene fluoride (PVDF), is bonded to the backside of the substrate (e.g. glass). The impedance characteristics of the piezoelectric elements, which are coupled to the substrate's thickness resonance, are monitored by electronics. A finger touch absorbs acoustic energy, damps the thickness resonance and hence alters the Q (quality factor) of the resonant system, and thus changes the impedance characteristics of the piezo coupled to the thickness resonance. This scheme thus employs the known damping of acoustic waves by an absorptive object, and does not employ a scattering structure or grating.